1. Field of the Invention
This invention relates to a process for separating isomeric diaminodiphenylmethanes and is more particularly concerned with the isolation of substantially pure 4,4'-diaminodiphenylmethane from admixture with the corresponding 2,4'-isomer.
2. Description of the Prior Art
4,4'-Diaminodiphenylmethane is a well-known compound which is useful as an intermediate in a number of reactions. For example, it is an intermediate in the preparation of polyamides such as nylon and in the preparation of the corresponding 4,4'-methylenebis(phenyl isocyanate) which latter is widely used in the preparation of polyurethane elastomers. The diamine is normally prepared by condensation of aniline and formaldehyde to yield a mixture of polymethylene polyphenyl polyamines in which the major component (40% by weight or higher) is diamine and from which the latter is isolated by, for example, distillation. The diamine so prepared is generally a mixture of the 4,4'-isomer in association with varying amounts of 2,4'-isomer and some 2,2'-isomer depending upon the particular method employed in its manufacture; see, for example, U.S. Pat. Nos. 3,362,979; 3,676,497 and 3,857,890.
For the various uses of 4,4'-diaminodiphenylmethane discussed above it is generally necessary to employ the compound in a substantially pure form. By "substantially pure" is meant that the diamine contains less than about 5 percent, and preferably less than about 2 percent, by weight of the corresponding 2,4'-isomer and or 2,2'-isomer. Various methods of achieving such a purity have been described. A commonly used method is fractional distillation under reduced pressure which normally involves some overall loss of material due to decomposition, polymerization and the like reactions which occur due to prolonged exposure to heat in the distillation kettle.
Canadian Pat. No. 745,173 describes the isolation of 4,4'-diaminodiphenylmethane in substantially pure state from the crude reaction product derived by condensation of aniline and formaldehyde in the presence of hydrochloric acid. The latter product is diluted with hot water and partially neutralized with alkali whereupon the substantially pure monohydrochloride of 4,4'-diaminodiphenylmethane crystallizes out upon cooling of the product. Because of the large volumes of solution required and other factors, the process is not suitable for adaption to continuous commercial operation.
British Pat. No. 1,169,127 describes the separation of 4,4'-diaminodiphenylmethane from admixture with its isomers by treating the mixture with an alkali metal or alkaline earth metal halide, cyanide or isothiocyanate, with which the 4,4'-isomer selectively forms an adduct, separating the adduct so formed, and regenerating the 4,4'-isomer from the adduct by heating with water or an inert solvent. Removal of last traces of the salt used to form the adduct is one of the drawbacks of this procedure which is not readily adaptable to commercial operation.
We have now found that the 4,4'-isomer of diaminodiphenylmethane can be readily separated from the corresponding 2,4'- and or 2,2'-isomers by a simple procedure which is readily adaptable to commercial production and which is free from the disadvantages noted above.